Currently Being ModeratedDec 9, 2012 9:56 AM (in response to Mac OS XP)
Before everything ends in tears... would suggest that each Mac has its Own Dedicated External Drive for TM backups...
Mac 101 Time Machine http://support.apple.com/kb/HT1427
On the MBP there is this... But not on a Desktop...
Time Machine Local Snapshots Laptops
More Info here > All about TIME MACHINE